1. Field of the Invention
The invention relates to hybrid electric vehicle powertrains with a generator that may function as an electric motor during reverse drive.
2. Background Discussion
A known hybrid electric vehicle powertrain with split power flow paths is disclosed in U.S. Patent Publication US 2006/50022469, which is assigned to the assignee of the present invention. That powertrain includes an electrical power source including a traction motor, a battery and an electric generator that is capable of functioning as a motor. An engine is a mechanical power source, and a planetary gear system establishes divided torque delivery paths between the electrical and mechanical sources of power and vehicle traction wheels. The planetary gear unit includes a ring gear connected drivably to the traction wheels, a sun gear drivably connected to the generator and a carrier drivably connected to the engine. The generator, the motor and the battery are electrically coupled.
Engine torque applied to the carrier during forward drive in a divided power delivery mode is in a direction that is opposite to the direction of reaction torque applied to the sun gear by the generator. The torque applied to the carrier by the engine is equal to the sum of the torque applied by the engine to the ring gear and the torque applied to the sun gear.
If the electric motor is used during an electric motor launch with the engine off, the motor will drive the ring gear in the same direction as the direction of rotation of the motor rotor. Since the carrier is directly connected to the engine, which is not fueled at that time, the sun gear rotates relatively freely while the carrier is not turning. The planetary gear unit at that time is essentially without a reaction element, except for a small torque delivered to the sun gear by reason of bearing friction losses and gear friction losses.
If the generator is commanded to provide assistance to the motor during a forward motor launch, the generator is controlled to function as a motor as it rotates in a negative direction (i.e., counterclockwise as viewed from the engine's location). At that time, torque is applied to the carrier in a negative direction. An overrunning coupling provides reaction torque at that time so that generator torque can be transmitted to the ring gear, which drives the traction wheels in a forward driving direction. The battery then provides power to both the motor and the generator. The known control strategy for the powertrain illustrated in patent publication US 2006/0022469, now U.S. Pat. No. 7,285,869, will not allow the generator to assist the motor during a reverse launch since there is no reaction torque available to allow generator torque to be distributed to the ring gear.
Since positive engine torque (i.e., clockwise torque as viewed from the engine's location) will reduce the drive torque contribution from the motor, the known control strategy executed by the vehicle system controller will minimize the use of the engine during a reverse operation. Under these circumstances, there is a limited operating range in which the engine can be used to supply driving torque to the generator to charge the battery when the battery does not have a sufficient state-of-charge to supply power to the motor.
A peak reverse wheel torque is an important factor that determines a vehicle's ability to climb a steep grade, or to maneuver the vehicle in reverse in certain off-road conditions, or to maneuver the vehicle in reverse over obstacles, such as a roadway curb. A powertrain of the type disclosed in U.S. Pat. No. 7,285,869 can develop only a limited peak reverse drive wheel torque.
Reverse gradability performance could be improved by making hardware changes, such as by increasing the current rating of the power electronics. Further, motor torque at low speeds could be increased at the expense of power at higher speeds by changing the rotor and stator configurations. Still further, gear ratio changes between the motor and the traction wheels could be modified for more reverse wheel torque. However, such hardware changes can lead to undesirable compromises in cost, performance and durability. For example, increasing the current rating of the power electronics will increase the cost of the powertrain. Also, changing the rotor and stator configurations would affect acceleration performance at higher vehicle speeds. Finally, increasing the gear ratio between the motor and the wheels would lead to reduced powertrain durability by aggravating powertrain instability during driving events that involve, for example, anti-lock braking and traction control.